Girvanella Stromatolite piece (Oncolith Nodule) ± 9 cm in diameter.
* Oncoliths are sedimentary structures made up of oncoids, which are layered structures formed by the growth of cyanobacteria. Oncoliths are very similar to stromatolites, but instead of forming columns, they form roughly spherical structures. Oncoids often form around a central nucleus, like a shell fragment, and fouling microbes deposit a calcium carbonate structure. Oncoliths are indicators of warm waters in the photographic zone, but are also known in contemporary freshwater environments.
Paleocene, Flagstaff Limestone
Near Thistle, Utah (USA)
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